Public health officials say 24 unvaccinated students have been sent home from an Orange County high school because of the risk a student infected with measles exposed others to the highly contagious respiratory illness.

Small but unusual outbreaks of tuberculosis, measles and an extremely rare polio-like disease in several regions across California have set off concerns about the risks of exposure. All the infectious disease reports are actively being investigated, and California health officials say it is sheer coincidence that they arose at the same time.

Democratic Sen. Barbara Boxer is asking the Centers for Disease Control and Prevention to initiate a formal investigation into what has caused polio-like paralysis in about 20 children in California over the past 18 months.

A growing shortage of medications for a host of illnesses — from cancer to cystic fibrosis to cardiac arrest — has hospitals scrambling for substitutes to avoid patient harm, and sometimes even delaying treatment.